Add comprehensive SSH key management and validation features

- Add support for both private and public key storage
- Implement automatic SSH key type detection for all major formats (RSA, Ed25519, ECDSA, DSA)
- Add real-time key pair validation to verify private/public key correspondence
- Enhance credential editor UI with unified key input interface supporting upload/paste
- Improve file format support including extensionless files (id_rsa, id_ed25519, etc.)
- Add comprehensive fallback detection for OpenSSH format keys
- Implement debounced API calls for better UX during real-time validation
- Update database schema with backward compatibility for existing credentials
- Add API endpoints for key detection and pair validation
- Fix SSH2 module integration issues in TypeScript environment

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/backend/ssh/tunnel.ts

@@ -455,7 +455,7 @@ async function connectSSHTunnel(
         const credential = credentials[0];
         resolvedSourceCredentials = {
           password: credential.password,
-          sshKey: credential.key,
+          sshKey: credential.privateKey || credential.key, // prefer new privateKey field
           keyPassword: credential.keyPassword,
           keyType: credential.keyType,
           authMethod: credential.authType,
@@ -501,7 +501,7 @@ async function connectSSHTunnel(
         const credential = credentials[0];
         resolvedEndpointCredentials = {
           password: credential.password,
-          sshKey: credential.key,
+          sshKey: credential.privateKey || credential.key, // prefer new privateKey field
           keyPassword: credential.keyPassword,
           keyType: credential.keyType,
           authMethod: credential.authType,